How many fstops does a hoya R72 ir filter cut?

Discussion in 'Accessories' started by al_n., Oct 12, 2015.

  1. Hello, I have been experimenting with ir filters for a short while, and I cant seem to get replicable shots. My question is, from a base exposure on my camera without the filter, how any fstops will it cut on my camera? Thanks for any and all replies!
     
  2. As the R72 is an IR filter, cutting visible light below 720 nm and passing IR above this, as shown here:
    http://www.bhphotovideo.com/c/product/23077-REG/Hoya_B55RM72_55mm_R72_Infrared_Glass.html#!
    exposure is going to be a matter not of visible light (which you can measure with an exposure meter) but of IR (which you can't). I can only recommend wide bracketing around a meter reading until you gain experience of what works.
     
  3. William Michael

    William Michael Moderator Staff Member

    About 11 to 14 Stops I have found is a reasonable guess to put me in the playing field of a reasonable IR exposure from which I can make something worthwhile.
    Have a look at these, they might assists you. Each scene is at about EV = 14, but the lighting direction and type is different in each so as you will extrapolate the answer varies. The exposure that I used is below in the frame of each Photograph: view each photo "large" would be best.
    It probably will depend on the camera you are using, too and also the Post Production you perform. I did a reasonable amount of IR Film work years ago but only recently (a few months ago) started IR with a Digital camera. I am only using the Fuji x100s. Because I tend to be procedural and prefer to learn some things from trial and experiment (rather than instruction) - I started with one 'typical' front-lit scene and bracketed a gazillion brackets and then set about standardizing my Post Production procedure. My best results thus far are those three images - but if you look closely each is slightly different even though the Post Production was the same. Like all photography it's all about the light (well actually 'the heat') and that depends upon then time of day. That's why I have been playing around lately at sunrise, near the ocean and comparing and contrasting those results with midday and afternoon shots of the plants in my garden.
    WW
    Addendum: David and I were responding at the same time.
    Yes as David alluded: probably the technical correct answer is 'infinity', because and IR Filter should attenuate ALL visible light to zero.
    And also as David mentioned: lots of brackets - that's exactly what I did, as I mentioned, but I reckon I have put you near the goal posts.
     
  4. A lot depends on the effectiveness of the IR cut filter on your sensor, as well as the amount of IR in the scene. Trial and error is the only reliable method.
    If you have a camera you are willing to dedicate to IR photography, the IR filter can be removed and replaced with a low pass filter (removes visible light) for about $500. Even a camera with a weak IR filter (e.g., D100) removes 4-6 stops in daylight. An R72 blocks the viewfinder, whereas the conversion gives you an unobstructed view.
     
  5. Hi Al. If you're shooting film, this may help, and point out the sad loss of HIE. In digital, it very much depends on the ir frequencies present in the scene and the responses of the sensor and meter. BUT, if your camera supports live view (or is not an SLR), just use live view mode. Then it'll meter using the actual sensor and what it's seeing, rather than the separate meter's take on the scene, so you shouldn't have to worry about it. If that's not the case, just expose manually and eyeball with live view. And if the camera doesn't have live view, all I can suggest is bracketing. Unless you've had the ir filter removed, expect to have to increase exposure quite a bit, as others suggested - I remember seeing about five stops near noon on a D700, for example, with its internal filter fighting an R32. Good luck. You'll get better at guessing with practice, at least in terms of weather conditions.
     
  6. it

    it

    Quick check of the intergoogle says 8-10
    I can't see how it's much more, my Big Stopper and Little Stopper combined is 15 and there is no way it's in that range.
     
  7. Ian: I could believe 8-10 stops of most of the visible
    spectrum (I CAN see a light bulb, in red, through an R32,
    just - obviously NEVER try looking at the sun through
    one...) I suspect most camera sensor ir filters aren't
    quite this effective at blocking ir, since you can get a
    viable ir image from an unmodified camera using one.

    If Al's question was "I want to take a photo in infrared;
    how much more exposure will I need for a black subject
    that reflects in infrared compared with visible light?" I
    guess we were all actually taking more about a typical
    camera sensor's response to the ir light that gets
    through an R72 - and I stand by the suggestions that 5-6
    stops are a good starting point. If his question was "how
    much darker does an R72 make a white object that's
    black in infrared?" then your 8-10 stops figure seems
    plausible to me.

    Put another way, I could hand-hold daylight exposures
    on an unmodified D700, and I think I was at about ISO
    3200 (it's been a while) where I might normally have
    been at ISO 100. I certainly didn't need ISO 102400 (ten
    stops). But obviously camera sensors vary.
     
  8. Thank you all for the replies! William, those are some great shots. I tried similar camera settings to what you have and got some decent images. I shot the same scene several times, each time setting the exposure for a longer time. I started at 15 seconds, and increased the exposure by 5 seconds for each subsequent exposure. I am shooting with my Canon t3i/600D and I wanted to try film but am only doing digital for now. When I was taking the pictures I was using live view, and I did not think about just shooting at what the camera meter said to take. I thought that it wouldn't really help much, so I just took a bunch of bracketed shots. I am going to try shooting at the exposure that the camera tells me to, and see what results I get.
     
  9. Silicon is sensitive into the IR, so the meter should still give a reading. (This is also why the sensor works in
    IR.) CdS is blue sensitive, Selenium down to about 800nm.
    But the actual compensation from a DSLR meter reading to the image sensor reading might be somewhat different, as there is usually an IR blocking filter. The filter is needed for proper color rendering for sources of IR.
    If the meter doesn't have a matching IR block filter, it won't work exactly with IR, but usually close enough.
    I assume you have a digital camera, just keep taking until it comes out right.
     
  10. Glen: both the ir filter and any colour filter stay over the
    sensor will respond differently to the main meter, given
    different lighting conditions. However, with most
    cameras, live view uses the main sensor to judge
    exposure (I believe), so any exposure variation should
    sort itself out. Using a dslr optical finder and the
    separate meter is likely to be somewhat less predictable.
     
  11. For many years, we had selenium, and then CdS light meters, without worrying about the spectral response. Then sometime later, most switched to Si photodiodes, especially for in camera TTL on film SLRs. I don't know if any put an IR block filter over the Si sensor.
    Now that you mention it, I don't know at all how DSLRs meter. I might have suspected that they used separate photodiodes.
     
  12. Sorry about the late response, but wanted to clarify a bit.
    It depends on your camera and the amount of reflected light.
    Modern sensors - such as my D800E - have about 14-15 stops of filtering of IR light on the sensor. This is a tested result comparing exposure times on a converted IR camera with a filter to an unconverted camera.
    An IR filter such as the R72 blocks visible spectrum below 720nm - all the visible spectrum. But blocking is a relative term. If you are using a conventional camera in bright sun, you may get some light leakage through the viewfinder or the distance scale. Be sure to shade the camera and cover the viewfinder during the exposure to prevent light leakage.
    Live View and Auto Focus won't work with an R72 filter on a conventional digital camera. It works great on a converted camera.
    With a current camera in bright sun, I'd expect exposure times in the 15-30 second range at base ISO with an 81C filter (an 810nm filter that is for pure IR spectrum). You can increase ISO for a faster shutter speed. You'll have to try some test images and look at the histogram. You may have some light leakage in the red spectrum - some visible light - and that would shorten exposures.
    The R72 filter will allow a little visible spectrum to pass through, so I would expect a 2-3 second exposure as a starting point. The exposure is likely to be dominated by leaking red light from the visible spectrum since the sensor blocks 14 stops of IR and your filter does not block the 700-730nm range of barely visible spectrum very effectively. You want visible spectrum with a 720nm filter - it's just that it dominates IR on an unconverted camera. (Blocking 10 stops of visible light with an R72 filter is 99.9% - it's just that you still have 10 times more visible light than IR in your image. This is known as false IR since it is mainly red light.)
    There is a big difference in exposures with less than full sun. Subject matter does make a difference as it influence reflection of IR spectrum. Bright green foliage or synthetic fabrics are highly reflective of IR spectrum.
     

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